Here http://docs.oracle.com/cd/E14373_01/appdev.32/e11838/app_comp.htm#CHDJCDCF
in the "Populating Forms" paragraph, I find:

Oracle Application Express populates a form either on load or when the Application Express engine renders the page.

Can someone point me to a discussion/explanation of 1) what "populating" gets done at "load" and 2) what "populating" gets done when the engine "renders the page"? Are these all the "populating" steps then? Once a form/page is rendered, it can't be populated further -- or can it?

Oracle Application Express populates a form either on load or when the Application Express engine renders the page.

When a apex page is being rendered the APEX engine process them in a sequential order they are defined +(evaluating the conditions)+
You can see that using the edit any page > Utilities button > select Page Events.
See http://docs.oracle.com/cd/E37097_01/doc/doc.42/e35125/bldr_pg_def_about.htm#HTMDB28225

Can someone point me to a discussion/explanation of 1) what "populating" gets done at "load" and 2) what "populating" gets done when the engine "renders the page"? Are these all the "populating" steps then? Once a form/page is rendered, it can't be populated further -- or can it?

Once the page is rendered it will have all values populated as defined..but you can only populate further values using dynamic actions (which is AJAX).

I'm trying to get a model of how APEX handles data (values). There are several places(?) that data (values) can exist. Or so it seems. I'm trying to understand how these work -- to put all the pieces together.

Question: Where data can be?
1) In the database
2) In the session
3) Rendered -- and hence displayed on the screen, if a displayed value -- but not in the session
4) In perhaps(?) some working memory pool(?) but different from the page rendered values I see on the screen?
5) Other?

Why do I think this is relevant? If there are rendered values and session values -- different from the rendered values, then when code executes, which of these values (rendered, session) is it executing against?

And maybe the answer is, "You don't understand what's going on." To which I answer, "Exactly!" Hence the question.